This invention generally relates to a method of fabricating an MRAM structure, and more particularly to an MRAM structure that has a pinned layer formed above an insulating layer and within a protective sidewall.
Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, but indefinitely alterable, and consumes little power. Magnetoresistive random access memory (MRAM) technology has been increasingly viewed as offering all these advantages.
A magnetic memory element has a structure which includes magnetic layers separated by a non-magnetic layer. Information can be read as a xe2x80x9c1xe2x80x9d or a xe2x80x9c0xe2x80x9d as directions of magnetization vectors in these magnetic layers. Magnetic vectors in one magnetic layer are magnetically fixed or pinned, while the magnetic vectors of the other magnetic layer are not fixed so that the magnetization direction is free to switch between xe2x80x9cparallelxe2x80x9d and xe2x80x9cantiparallelxe2x80x9d states relative to the pinned layer. In response to parallel and antiparallel states, the magnetic memory clement represents two different resistance states, which are read by the memory circuit as either a xe2x80x9c1xe2x80x9d or a xe2x80x9c0xe2x80x9d. It is the detection of these resistance states for the different magnetic orientations that allows the MRAM to read and write information.
In standard MRAM processing, there are certain sensitivities related to the use of optical photolithography. Typically, the free magnetic layer is patterned separately from a previously deposited copper interconnect line and the pinned magnetic layer, which rests over it. This separate patterning requires a photo-step, in which registration is critical for placement of the free layer over the pinned layer.
Spin etching is typically used to form the pinned layer. Spin etching causes the pinned layer to be xe2x80x9cdishedxe2x80x9d or recessed in the center to a greater degree than the more exterior regions. This recessed shape is desirable because it is thought to cause more of the electromagnetic field to be directed at the free magnetic layer, thereby reducing the current needed to change the state of the free layer. Spin etching is notoriously non-uniform as it relates to the variations between the center and the outer regions of the wafer. Additionally, there are problems wit h lopsidedness at the trailing edge of the spin caused by this process.
It would be desirable to have a method of fabricating the MRAM structure whereby the structure is formed in a more accurate and reliable way. Sidewall protection of the MRAM structure, prevention of copper migration, and accurate definition of the structure are all characteristics desired to be improved. Additionally, processing of the MRAM structure without need for spin etching so as to achieve a more uniform structure across the wafer would also be advantageous.
This invention provides a method of fabricating an MRAM structure. The MRAM structure of the invention does not have the pinned layer recessed within a trench, but instead forms it above an insulating layer. The method provides a sidewall protection for the bottom magnetic layer of the MRAM structure and insures a more reliable structure, which also allows definition of the MRAM stack by a self-aligning process. By this self-aligned process, the bottom portion of the MRAM stack, incorporating the bottom magnetic layer, is defined in a single etching step and the top portion, incorporating the top magnetic layer, is defined above the bottom magnetic layer in another single, self-aligned etching step, which positions the top magnetic layer over the bottom magnetic layer.
This process allows for the fabrication of MRAM structures without employing trench process technology. It eliminates many of the sensitivities associated with optical photolithography as well as the process variabilities associated with spin etching of the recess region for the pinned layer. Finally, it allows for accurate control of the top magnetic layer in its positioning over the bottom magnetic layer so as to improve the electrical characteristics of the MRAM.